Method and apparatus for medium high frequency direction finders



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METHOD AND APPARATUS FOR MEDIUM HIGH FREQUENCY DIRECTION FINDERS Filed July 19, 1943 6 Sheets-Sheet 4 V V .M l 55'; r i l I INJECTION E I VOZTAGF MAYER u/v/r immoral? I a use/4mm? AEFfREA/(F razT/w-E I Jan. 22, 1946. w, wlRKLER 2,393,353

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,4 TTOAA/-V wa /(aw 36 Fatented Jan. 22, 1946 METHOD AND APPARATUS FOR IWEDIUM men FREQUENCY DIRECTION FINDERS Walter H. Wirkler, Cedar Rapids, Iowa, assignor to Collins Radio Company, Cedar Rapids, Iowa, a corporation of Iowa Application July 19, 1943, Serial No. 495,339

18 Claims.

pair antenna medium high frequency direction finder systems to substantially eliminate polarization errors.

Another object of my invention is to provide a three antenna pair system for medium high a frequency direction finders in association with a pilot control signal system and an automatic balancing circuit.

Still another object of my invention is to provide an improved multiple pair antenna medium high frequency direction finder system in which errors due to diversity eifect may be substantially eliminated.

A further object of my invention is the 'use of the heterodyne direction finder circuit in which each antenna element of a multiple pair antenna system is connected individually to a separate receiver with circuit provisions permitting the use of a single pair of receivers with three antenna pairs.

A still further object of my invention is to make available indicating voltages derived from each antenna pair of a multiple pair antenna system simultaneously for the operation of the indicating circuits.

Another object of my invention is to provide means for deriving the average -or integrated value of a fluctuating bearing indication by means of integrating devices associated with each indicating voltage circuit in a multiple pair antenna system radio direction finder.

Stillanother object of my invention is to provide several integrating devices in each indicating voltage circuit for separately integrating bearings from several transmitters sharing time on the same frequency in a multiple pair antenna system radio direction finder.

A further object of my invention is to provide bearing indicating means actuated by the indicating voltage. In particular, circuits are provided for actuating a two-circuit cathode ray indicating device from three indicating voltages in a multiple pair antenna system radio direction finder.

A still further object of my invention is to provide marking means for the cathode ray trace to identify the sense of the indication in a multiple pair antenna system radio direction finder.

Another object of my invention is to provide an improved electronic commutating circuit arrangement for a medium high direction finder system in which a pair of receivers may be connected with three pairs oi antenna systema- Still another object oi my invention is to provide commutating means for applying a pilot signal to the horizontal feeder in each antenna pair in the proper sequence for detecting antenna circuit unbalances.

Other and further objects of my invention reside in the improved balanced circuit arrangement for medium high frequency direction finders as set forth more fully in the specification hereinafter following by reference to the accompanying drawings, in which:

Figure 1 is a theoretical view used in explaining my invention; Fig. 2 is a curve diagram illustrating the characteristics incident to the divers'ity effect which may cause serious errors in a two pair antenna system and which is materially reduced in the three pair antenna system in my invention; Fig. 3 is a schematic diagram showing the distribution of currents in the structure comprising one pair of antenna elements and the associated feeders employed in the system of my invention; Fig. 4 is a schematic illustration showing the arrangement of one antenna pair with respect to the central equipment together with the pilot signal inducing means employed in the direction finder system of my invention; Fig. 5 is a theoretical view showing the direction of the pilot-signal excitation to the three antenna pairs during the three parts of the commutating c cle; Fig. 6 is an illustrative view of the equivalent electrical circuit of the horizontal feeder structure of one antenna pair used in calculating the distribution of pilot signal current and current induced by the horizontally polarized Wave in the direction finder system of my invention; Figs. 7, 8, 9 and 10 condointly disclose, when aligned edge to edge, the circuit arrangements for the medium high frequency direction finder system of my invention; Fig. 11 is an enlarged view of the interconnection means for the three pair antenna systems and the pilot conductors used in the system of my invention; and Fig. 12 is a diagram matic view of the antenna amplifier employed in connection with each of the dipole antennas em ployed in the system of my invention.

Direction finding technique for low frequency ground waves is generally well understood because the received wave has only one component, with a constant polarization. To a large extent, this is true also of ultra-high frequency waves at short distances. At medium high frequencies beyond the range of the direct or ground wave, the conditions of polarization, vertical angle of arrival, and multipath reception are never constant and give rise to various design and operation problems which have never been very well understood. Often the errors of various kinds have been over estimated, under estimated, or ascribed to the wrong cause. The result has been much confusion and misunderstanding as to the ultimate possibilities, the design requirements, and

the operating technique for direction finders in the frequency range 2 to 20 mc./sec. 4

My invention is directed to an accurate direction finding equipment built for this frequency range which will provide accurate bearings with low power over long distances a large percentage of the time.

In rotatable equipment two spaced collectors, the vertical angle of arrival does not cause errors directly but does increase the effect of polarization error by decreasing the relative directional sensitivity to the norof the type involving :to keep all errors -time required for mally polarized component as compared to the sensitivity to the components giving rise to polarization error. Multipath reception over several vertically-displaced paths similarly causes no error with this equipmen Multipath reception involving paths laterally displaced from each other naturally causes errors in rotatable as well as in fixed antenna equipment. v When the major component of the received energy arrives along the great circle path and a minor component arrives along a laterally-dis? placed path, the indicated bearing swings back and forth as the phase relation between the two waves changes. If the rotatable direction finder is oriented so that the great circle wave produces an on-bearing indication theweaker laterally-deviated wave may, for a particular phase relation, cause the indicated bearingto be deviated to the right. If the phase relation between-the two waves now undergoes a' change of 180, the indicated bearing'will. be deviated to the left. If an integrating circuit is applied to the indicator, the indicated bearing will integrate to the great circle value over a period of time.

Polarization error produces a displacement of the indicated hearing from the great circle similarly dependent on the phase relation between the normally polarized component and the abnormally polarized component. The integratin circuit would eliminate polarization error just as it eliminates the effect of a minor laterally-displaced wave. Since the phase relation between the laterally-displaced wave, the abnormallypolarized wave, and the principal great circle wave is purely random, it is impossible to tell which of the twoefi'ects is responsible for the observed swinging. Since the eflect of the laterallydisplaced wave is beyond the control of the tie signer, he must design the equipment so that polarization errors are 'much less than the effects of lateral displacement to be certain that the swinging observed will not be confused with polarization error. Although the improvement in performance obtained by a reduction of polariza- "tion error may be masked by the lateral deviation reflect so that effort expended in this direction does mandatory that the aural null type, to be classed as a ascasss tween displaced waves. Although the true bearing will be obtained by integrating over a nifllciently long period or time, it is important that thisxdiversity efiect be held to a minimum because it can be at least as-serious as polarization error.

While the integrating circuit averages out all of the errors mentioned above, it is still desirable to a minimum'to reduce the taking a bearing. Since it is possible that some signals will be relatively free of thesecondary lateraldeviation effect and 'yet exhibit polarization and diversity efiects, the lattertwo must be held to a minimum. In general, however, the integrating time required to take an accurate bearing will be determined by the secondary lateral deviation eiiect, which is beyond the control of thedesigner. The best he can do in this respect is to make the integrating convenient and of adjustable time constant.

In the case ofaural null direction finders, the integrating is never convenient because it involves taking a series of bearings, and averaging them, This involves continuous and very tiresome swinging of the direction finder. Hence it is almost direction finder be of the visual indicating type. if the full advantage of integration is to be realized.

Although a rotatable direction finder or the visual indicating type is quite an improvement over the aural null direction finder-from the standpoint of convenience in integrating, it is, like the "hunt and guess system in the presence of swinging signals. In other words. the operator first sets the direction finder at what he guesses to be an on-bearing heading and allows it to integrate our sev-' eral swing cycles. Thevisual indicating circuit informs him that his azimuth is too low, so he 40 tries a higher azimuth and waits to see what he gets. The visual indicating circuit may then tell him he is too high. and so on'until he finally gets an. answer that looks about right. If, however,

. the station has stopped sending in the meantime,

when it comes on again a few seconds later he has forgotten where he was and must start all over. The long time required i'or taking a hearing is a result of the fact that the integrating circuit wastes its time whenever the direction finder heading is very much oil-bearing and makes good use of its time only when the heading 'not seem to produceproportional returns,'it is still the part of wisdom to design the equipment with an absolute minimum of polarization error.

a In addition to errors of the two types mentioned above, equipment of the fixed antenna type is subject to another group of errors related to the vertical angle of arrival. The most serious of these errors is a result of interference between two is so nearly correct that an approximate on-bearing indication is obtained.

In the case of automatic indicating equipment of the fixed-antenna type, this difliculty is not present because the apparent bearing is indicated at all times and does not depend on any folIow-up'action on the part of the operator. when the bearing is swinging, he may follow the indication by eye and mentally determine an integrated bearing. However, such mental integration. isnot very reliable because the mind is easily confused and may give undue weight, to a large excursion in one direction, or to stemplace.

or more waves arriving over vertically-displaced paths in the plane of the great-circle. Hence these errors will be included under the general classification of diversity effect. -In the case ofinterference between two vertically-displaced waves the deviation of the indicated hearing from the true bearing depends upon the phase relation between the two waves just as it dbes in the case of polarization error and interference be- The obvious mission to this problem is to make the equipment automatic integrating as well as automatic visual indicating, as in the design to be described. If the integrating time constant is adjustable, the direction finder can be operated normally with a low time constant for taking snap hearings on signals steady enough to permit. or on signals or such short duration that snap bearings-.are necessary. If the signal persists and is difierent vertical angles. 7 ever, that at the lower frequencies thereis never received wave is laterally deviated from the great circle path, much of the time the major component is on the great circle and the swinging due to minor laterally-deviated components can be integrated out.

Equipment of this type has not been built in the past partly because no fixed-antenna equip- .ment haseverwbeen free ofdiversityefiect or polarization error over a wide range of frequencies, and partly because too much emphasis has been placed on "instantaneous bearings. In other words, since polarization error and diversity eifect have never been small, it has been hoped that if these effects could be eliminated, snap bearings could always be obtained. .Once equipment sumciently free of polarization error and diversity effect is available and it can be shown thatswinging due to secondary lateral deviation still exists,

it will be realized that optional automatic integrating circuits are a necessit for accurate work part of the time, and more attention will be given to their development. This should result in a revival of interest in medium high frequency direction finders for long range and emergency naviga tion purposes as well as for military intercept uses. It is believed that fast and accurate bearings will be obtained a much larger percentage of the time than is now generally thought possible.

The three major problems in the design of a fixed-antenna direction finder of optimum performance then are:

I. Reduction of diversity effect. II. Reduction of polarization error. 111. Design of automatic indicating circuits with adjustable integrating features.

I. Diversity efiedt Control of diversity efiect has usually been attempted by. reducing the spacing of the collectors.

Forv various reasons, this results in loss of performance from the standpoint of sensitivity, tuning range, accuracy of indication, and polarization error. This method therefore will not be discussed any further.

Another proposal has been the use of horizontal loops instead of vertical dipoles as collector antennas. The chief argument for this method is that the most extreme cases of diversity effect result from interference between a high-angle sky wave and a ground wave. Since the horizontal loops are insensitive to the ground wave, this interierence will be eliminated, although not so much improvement is expected for conditions of interference between two sky waves arriving at It will be found, howany appreciable vertical magnetic intensity at reasonable heights above the ground, and the sensitivity of the device will be poor. Further, pure ground wave signals will be passed over altogether. It has been found also that the vertical componentof magnetic intensity which excites the horizontal loops, is adversely affected by irregularities in ground conductivity and that generally poor operation results.

From a practical standpoint, the design of such an antenna system will be rather troublesome, because loop antennas have no appreciable pickup factor unless they are tuned, and tuning introducs a variety of design problems because of gauging, mutual inductance, and phase stability considerations, especially when there are more than two collector antennas. For a practical design, then, let us revert to the vertical dipole collectors, analyze the cause of diversity efiect, and see what can be done about it.

Fig. 1 shows one collector pair, with the line joining the two antennas making an angle a with a horizontal line in the wave front. The dimension X in Fig. l is a measure of the distance spanned by the collector pair perpendicular to the wave front. X is considered to be positive or negative, depending on whether e is positive or negative. In fact, X =s sin a, where s is one-half the collector spacing.

Consider a two-pair (four-collector) direction finder in which a circuit isused to produce an indicated bearing 4 2 0 tan yb where ya is a. voltage derived from antenna pair A and Zlb is a voltage derived from antenna pair B. Pair B is erected at right angles to pair A. The indicated bearing will be accurate onlyif ya=KXa, and Zlb=KXb, since y., K rain 6 tan a E K s cos 0 This requires that the derived voltage 3 (labelled phase-meter output) be related to X by the straight line relation 1 of Fig. 2.

Failure to realize the straight line relation between ;1 and X is a most serious source of error in fixed-collector direction finders, and is due chiefly to three causes:

=tan 0 l. Octant effect. 2. Diversity effect. 3. Non-linearity of indicator circuits.

The so-called octant effect is a result of the fact that the output of one antenna pair is not directly proportional to the phase difference, Z, between the voltages induced in the two collectors, but to their vector difference given by 11=2E sin ,4 2, where E is the magnitude of the voltages induced. This relation resembles curve 2 of Fig. 2 and gives rise to octant error, so called because it has a maximum value in eight directions on the compass. Octant error is relatively small for reasonably close antenna. spacings and canbe calibrated out if the vertical angle of arrival is known approximately.

Diversity eflect is caused by interference between two or more waves arriving at difierent vertical angles and is so named because it gives rise to fading cycles which are different at an tennas horizontally spaced from each other. This effect has been employed in diversity receiving systems to reduce the effects of fading. It is obvious that no diversity effect can exist between antennas located in the plane of the wave front (9:0, Fig. 1), because each of the vertically-displaced waves arrives at one antenna with the time phase at which it arrives at the other, so that the two fading cycles are synchronous. For this reason, no diversity error exists in the rotatable direction finder when on-bearins, and

the diversity error when slightly cit-bearing is very small.

In the fixed-collector direction finder, however, this diversity eflect may give rise to 1! vs. x

curves such as 2 or 3, Fig. 2, and can cause quite serious errors in the two-pair system. It is important to note that the 11 vs. a: curves obtained under diversity conditions are always symmetrical about X=0. That is. if X is made negative and positive by equal amounts the positions of the two collectors, as measured from a reference line in the wave front, are interchanged and y will be positive and negative by equal amounts.

Thus diversity eil'ect can give X vs.'1( curves such as 2 or 3, Fig. 2, but never such as I.

Non-linear indicator circuits also can give to any of the curves in Fig. 2. While it is comparatively easy to avoid non-symmetrical curves such as 4 by use of push-pull circuits and other circuit devices, curves such as 2 are usually more difflcult to avoid.

It is found that errors caused by octant effect,

rise

diversity effect, and non-linear circuits resulting in curves such as 2 or, I. Fig. 2, can be reduced greatly if three pairs of collector antennas are used instead of two. The three pairs are erected in 120 relationship to each other and the indicator circuits arranged to give an indicated bearing Y yb+!u ya I \i 3 (III: 110) where lla, 11b and 110 are the indicating voltages derived from the three antenna pairs, respectively. If all three voltages obey. the relation y='KX, as in 1, Fig. 2,

y,,=-KX.=Ks sin 0 fiery.)

The indication for the three-pair system is thus correct when the system obeys a linear law.

It can be shown that if curves 2 and 3 can be expressed as y=H+KX+LX=, where H. K, and L are arbitrary constants either positive or negative,J;he indication will still be correct:

116+ i ls 11a Whit-'11.)

h lT cos 20+sin 0) 34% cos a sin 0+sin' a] 5 flows on the sheath of the feeders of the other Since, in the relation v=H+Kx+Lx=, the con stants H, K, and L are purely arbitrary, this relation will fitmost curves of types 2 and 8, Fig. 2. The only requirements are that there be no even-powered terms of X, and no fiith or higher odd-powered terms. The first requirement is met by symmetry about the point X=0. The

second requirement is essentially met in practice because, with reasonable antenna spacing, the curve will be reasonably smooth so that no fltthpowered term is required to fit the curve accurately.

It is thus seen that the use of a three-pair collector system cancels out practically all of the octant error. diversity effect, and error dueto non-linearity which is present with the two-pair system, and the additional equipment required is well justified.

II. Polarization error Polarization error is caused by the fact that the relation between voltages induced in the various collectors changes with changes in polarization. In a. direction finder designed to operate from the vertically-polarized component, it is not essential that the collectors pick up no horizontally-polarized component, for example, but only that they have equal sensitivity, in magnitude and sense, whatever the state of polarization. This is the case in the spaced vertical loop rotatable direction finder in which the two loops are sensitive to both vertically and horizontally polarized waves arsiving'trom certain directiem,

but, since the loops are in parallel planes, the relation between the vertical and horizontal pickup factors is the same in the two loops.

For fixed-collector direction finders, however, non-directional collectors are more desirable. 'Of these, the vertical dipole is sensitive to the vertical component of electric intensity, and the horizontal loop is sensitive to the vertical component of magnetic intensity. In both cases, if the geometry oi. the installation is reasonably exact, the principal polarization error results from unwanted voltages induced in the horizontal feeders getting i to the'system through antenna unbalanceor re-radiation. It would seem at first thought that the horizontal p system might be inherently free of polarization error because the feeders pick up the same voltage the loops do. This is not the case, however, because the feeder sheath, considered as asingle conductor, lssensitive to the vertical, or parallellypolarized component of E from a wave arriving ata high angle. The direction of arrival is such that one 'collector pair is in-the plane of the wave front and, to'produce a proper indication. should have no output. However, current pair as a result of the vertically-polarized component of a high-angle wave. This current is strongly coupled to the collectors of the first pair in such sense as to be additive in the output circuit, and a large polarization error results.

In the case of the buried U type of vertical antenna system, polarization error is entirely a function of ground conductivity. A horizontallypolarized wave will set up earth currents at right angles to the direction of wave travel. If the soil is of finite conductivity, the vertical component of magnetic flux will penetrate some distance into the soil. The horizontal voltage induced by this flux is just equal to the voltage drop in the soil if the soil is sufiiciently uniform over a large area about the direction finder site. If the discontinuity in soil conductivity presented by the feeders themselves can be neglected, there will then be no horizontal voltage gradient in the soil and consequently no polarization error. If the soil is not uniform, however, or if it is of low conductivity so that the feeders introduce an appreciable discontinuity, a ground screen or a small system of radial wires is usually ineffective because the balance between induced voltage and voltage drop in the soil is upset by the ground screen itself, unless it is a solid screen of, say,

. hardware cloth of ver great extent, so that the current flowing in it near the direction finder site has no relation to the current flowing into it from the soil at its outer edges. Long buried feeder extensions are sometimes used instead of the complete screen for economic reasons. The system is practical only for certain permanent installations where the feeders can be buried deeply in uniform soil of high conductivity.

The elevated balanced collector system of Fig.

3 is preferred from the standpoint of ease of erection, high pick-up factor with untuned antennas, and sensitivity over a wide tuning range,

provided it can be, erected close to the ground and yet made free of polarization error. The principal cause of polarization error in vertical collectors of the balanced elevated type is the induction of current in the feeder sheath by the horizontally-polarized component. This sheath current gets into the antenna system either through capacity unbalance between the upper and lower limbs of the antenna, or by re-radiation and reflection from the ground as shown in Fig. 3.

If antenna unbalance were the only source of polarization error, it would be possible to eliminate this eflfect by balancing the antenna. Inserting series capacity in the lower limb should provide a balance because, if the antenna is reasonably short, it will act as a constant capacitance over a wide frequency range. Many experimenters, on finding that a perfect balance cannot be obtained and that the balance does not hold over a wide frequency'range, have concluded that the antenna has, in spite of its short length, a resistive and reactive component depending on frequency in a very complicated way. Actually, the trouble is probably caused by the fact that the mutual impedance between antennas depends strongly on their height above ground. Thus, the currents in A and A, of Fig.

from the theory of images as illustrated in Fig. 3, by considering the effect of current in the images inducing unequal voltages into the two antenna circuits, Either theory would explain why the elevation of the system above ground must be much greater than that dictated by consideration or capacity unbalance before a marked reduction in polarization error isachieved.

It would be desirable if the collector system could be erected comparatively near the ground. This would simplify the structure, make possible more speedy erection and provide security against observation. An additional advantage of erection near the ground is the substantially lower intensity of horizontally-polarized components. For these reasons, and to reduce Polarization error more than has been done in the past, an automatic balancing circuit, as described below, is recommended.

In the automatic balancing circuit, a pilot signal is induced longitudinally into the sheath of each horizontal feeder. If an unbalance exists, or if sheath current is coupled into the antenna system in any way, the pilot signal will'be heard in the receiver output and can be utilized to actuate automaticbalancing circuits which will neutralize this undesired coupling,

Fig. 4 shows how the pilot signal excites the main horizontal line longitudinally by means of a coil coupled to a portion of the main line formed into a horizontal loop. Coil and loop are surrounded by a shield box to eliminate direct induction of the pilot signal into the antenna circuit, but the main line leaves the shield box through an insulating grommet to permit induction of the pilot signal longitudinally.

Before proceeding with the design details of the automatic balancing circuit, let us analyze the necessary conditions under which it is capable of reducing polarization error. These conditions are:

I. That there be no other cause of polarization error than the eflect of currents in the horizontal feeder sheath set up by the horizontallypolarized field.

II. That the pilot-signal current flowing in the sheath have the same coupling into the signal line through the antenna system as does the current set up in the sheath by the horizontally-polarized field.

antennas were not plumb, so that the horizontally-polarized field could induce a voltage directly into the antennas.

The requirements for condition I are similar V to those for other types of direction finders except the buried U type of vertical antenna system. For example, power lines can be brought into the equipment by burying them to a sufil- -cient depth if the soil is of reasonably high conductivity. Alternatively, the power line may be in shielded cable laid on the ground radially with the shield connected to ground rods at intervals,

and with radial wires, similarly connected to ground rods, laid on the ground to maintain vertical loop rotatable direction finder installed near the ground. However, because the direction finder under discussion lends itself readily to semi-portable design, these requirements vshould receive more attention, inasmuch as sites with sandy or rocky soil will be encountered and it may not always be feasible to dig a trench for the power cable even when the soil is of high .conductivity. In some applications, it may be desirable to operate from batteries or an enginedriven generator and dispense with power and communication circuits entirely.

Condition II is necessary because the action of the automatic balancing circuit is only to neutralize the coupling of pilot-signal current into the antenna system, and it is necessary that this action also result in neutralizing the coupling of the current set up by the horizontally-polarized field. This means, essentially. that the current on the horizontal feeder sheath set up by the pilot-signal induction must vary along the sheath in exactly the same way as does the current set up by induction from the horizontally-polarized field, because the current anywhere along the length of the sheath is capable of inducing a voltage into the antenna circuit, while only that at the outer end of the sheath is impressed upon tube circuit 6 of Fig. 12 and transmitted to the automatic balancing circuit.

It is found, however, that the principal difference in the shape of the current distribution curves for the pilot-signal current with lumped induction, as in Fig. 4, and the current resulting from uniform induction bythe horizontallypolarlzed field lies in the ratio of 10, the current at the center, to L, the current at the end of the feeder sheath. In other words, if

mately sinusoidal in shape, with maximum cm'- rent at the center.

In Fig. 6 the horizontal sheath is represented as a uniform linev with characteristic impedance I 008 p tan are in n r z L 'cos a "N- 1 of 6-. Hence a-s=tan- ZwC'., where w is the I cyclic frequency. The voltage induced longitudinally by the horizontally-polarized field at a point'h is given by Ee dh, where E is the horizontally-polarized field intensity in volts per meter, 5:2

where x is the wavelength in meters, h is in meters, and N=sin 0, where 0 is the angle between the axis of the line'and the wave front, as in Fig. 1. If h is given in radians, the induction will be Ee I B l With all distances in radians, the current at h clue to the induction at h is v Ee mdh ""vsziaan F cot a--h)] Neglecting the resistance and radiation resistance'of the line, the current at 12 due to the induction at h is.

di,=:% %E-% dt., for h p and is cos p dz; for h p The total current at p is then A- 0 li-n 1 sm a--p cos p m' a-h It osTh b-o h-p Thisbecomes p I E sin (or-p)! a Jh JBZ sin (a-h) [tan h-cot (a-h)]" E .rzn cos p e Jpz, cos h [tan h-cot (ah)] 9 which reduces to p J E sin (a- )J n 1, B2. cos a cos he dh+ 8 M Jun .820 cos a 8111 (a h) e dh assasss On evaluating the integral, we have I sin 1N s+p) Bin (1N c'p 2 2 sin p cos (N+ )P tanacosp cos (N+1)s 2 N+1 This rather elaborate expression for Ip can be sin a in a simplified if we make use of the fact that any I, a 5" a '5 eifect which is symmetrical with respect to X, I" a Fig. 1, causes no error in the three-pair direction "2 a finder. Thus, since a e"=cos Nh+J sin Nh Q L ii and I. tan 0 cos a-sin s e- =cos Nh-J sin Nh which is the same as the sin terms reverse sign when N reverses sign, L1

andN reverses sign when 0 and X of Fig. 1 reverse I sign. Thus we may drop these terms, which for N= wing that the signifi n P r f he results in dropping the last five terms in the excurrent ratio is the same for distributed induction pression for h Evaluating the remaining terms when 0 is zero as it is for lumped induction half at p=o and p=s, we can write way out on the line.

tan a'sin (1=N)s' sin (I+N)-s] cos (I-FN)'s 2 sin %(l-N)s 1 2 l-N 1+N 1+N 1-N 1+N I, sin (a-s) sin (1N)s sin (1+N)s] 2 cos a 1-N 1+N which may be written It remains but to investigate how Law 15 tan 6 cos 8-8m 8 changes as 0 varies. For a line 3 meters from Where center to end; that is, 6 meters between dipoles 2 sin 2(1-N)s 1 cos (1+N)8 of one pair, with the characteristic impedance of 1- 1+ N the longitudinal line 400 ohms and Cs equal to F (1+N)8 sin mg I r 15 mini, the calculated values of 1; 1v *1 w so a gi g we Shall evaluate Fm for several frequencies are given in the table n w c below: I

F0 is F for N=0. 0=0 FM 13 F for N=V2| 9:309 85 Freq., mcJsec. N F .il s 0 and F1isFforN=1, 0 =90 g 0 8 i DWI; Mug This becomes as sIIIIIIIIIIIIIIIIIIIIIIIIII 1 01157 1185 is 33 8 5 i831:::::::::::::::::::::: 313% ii fi 3 3 38 3 y 10 1 0.313 1.878 as 90 B 5 i 33::::::::::::::::::::::::: 3Z5 :3 53 38 l 20 1 .683 1.88 72 so sin s+3 sin is and It will be noticed that the variation in F 1 cos 2s 1=- l 5m is comparatively small, particularly for the lower We wish to compare frequencies at which a small amount of spurious I pick up causes a relatively large bearing error. Further, the ratio changes appreciably only for values of a approaching 90, under which condiobtained. with induction from the 'horizontallytion the horizontally-polarized field does t Polarized field with duce very much voltage into the feeder. Also the I 4 output of the particular collector pair oriented I i 90 from the wave front has comparatively little obtained with lumped induction at the point eflect on the indicated bearing! all of which my be considered a fortuitous set of circumstances. The reason F is so nearly independent oi N,

. 2 despite the difierent form of the equations for as in Fig. 6. For this condition, F1 and F0, 'for example, becomes apparent from r 1 the following considerations:

-6 e5 lcos 2s h 'sin 2a+2s l -Writing the series expansion for the sine and I. am (as) cosine functions, and using 2s=L, we have 1., sin a-h 2-11 2. ;7=ag g -:-g 1-005 L 2 24 72o r.[ since. L+sinL Q L 4 720 4 this becomes v 75 F"; -723!- .1

For small values of a. corresponding to the lower frequencies, it is seen that F: is very nearly Comparing this with it is seen that F is very nearly the same for N= and N=1. For high frequencies, F is more dependent on N, but a I. is more independent of F. When the equivalent length oi line, including the capacity loading at the end, is a quarter wave. 0 becomes ninety degrees and 1. tans-F T. tan n cos s-sin s becomes I. cos:

- j independent of F.

From the analysis above it appears that the automatic balancing circuit is quite eiiective in It will be noticed that there is only one pair of receivers and one phasemeter circuit, and it can be seen from Fig. 7 that only one antenna pair is eiiective at any time. Aside from the saving in equipment, the advantage of this arrangement is that the outputs of the three antenna pairs are treated exactly alike. In'other words. in the relations of Fig. 2,

commute-ting cycle. 'This is accomplished byv means of the three condensers connected to the commutating switch in the phasemeter output circuit of Fig. 9. Each condenser is continuously connected to the grid of a tube in its particular branch, A, B or C, (Fig. 9) of the indicator circuit. Each condenser is connected to the phasebalancing antenna pick up in one antenna pair due to current induced in the feeder sheath of that pair by the, horizontally-polarized field. For current induced in the sheath by a high-angle vertically-polarized iield, the action oi the, automatic balancing circuitwill not be quite so effective, because maximum induction takes place when o=9.0' and N=1. Fortunately, however,

to 20 C. P. S., is sulflciently high so that the charge this eflect balances out in the three-pair direction finder, because the induction reverses sign as 0 reverses sign, so that the, output 01! the antenna pair due to this effect is symmetrical with respect to X, Fig. 1.

The effect or current in the feeder sheath of one pairon the antenna circuits or the other two pairs can be made to balance out also. From the equation for the indicated bearing.

I it can be seen that it the current the feeder sheath of pair a, for example. resulted inaflxed increment for the outputs, as and lie, or the other or thqother two pairs, there would be no eilfect' in the denominator, but-in the numerator the eflect on the indicated bearing would bethe same as though we and 11 were not changed but usuflered a corresponding negative increment,

. which would result in polarization error. To compensate for this ei'iect, it will be necessary, in connection with the antenna commutating scheme (Fig. 'l) to be described later, to apply pilot signal to pairs B and C as well as A while the automatic balancing circuit in A is energized, with the pilot signal excitation 01' B and C properly poled with respecttothat oiA.

The foregoing detailed analysis has been necessary first, to satisfy us that the three-pair collector system eliminates virtually all or the divarsity eiiect found in the two pair systemand, second, that the automatic balancing circuit can be made to eliminate practically all of the polarization error round in the elevated balanced-dipole system. It remains stillto investigate the problems connected with the design of visual indicating, integrating, and automatic balancing circuits lnapracticalinstallationmeter output circuit during the portion of the cycle that its particular antenna pair A, B or C, (Fig. '7) is energized. The commutating rate, 10

On the condensers reaches its ambient value after a few cycles, and a steady indication is obtained.

The condensers also serve the purpose of integrating a swingin bearing. It is found that if each of the voltages, 11a, 11s and ye is integrated separately, the integrated hearing will be indicated. Instead of a single condenser, each of the three filters in Fig. 9 consists of a group of condensers of diflerent capacitance so thatthe ca-.

pacitance of each group can be adjusted simultaneously by means of a gang-operated switch.

When taking a bearing on a telegraph signal with the smaller values of filter condenser, it will be found that the bearing indication gradually disappears as the charge on the condensers leaks oil. An optional circuit is provided in the phasemeterto key the indicating'voltage. This is accomplished'by means of a pair of neon tubes eflectively in series with the D.-C. phasemeter output circuit, so arranged that the tubes are ignited by the audio signal voltage. The filter condensers are thus eflectively disconnected from the phasemeter output circuit in the absence of signal, permitting them t retain their charge and maintain the indication.

The function of the indicator circuit is merely to apply the three voltages to an indicator unit in directions apart. Thus, if i is a horizontal unit vector and :i is a vertical unit vector, let 1 s and t be vectors 120 displaced from E Thus,

' Z=-' s=+i+z 4 t=+ii\/3 If we now apply Ila in the direction of 1 11s in the direction of s,.and ye in the direction of t, we have,

assesses which is equal to tan In other words the-directlon of the field is the indicated angle 0. v Fig. shows the circuit of my invention in which an A. c. voltage is obtained from each or three vacuum tube voltmeters proportional to the D. C. voltage applied to its grid. By means of a circuit similar to 9. Scott connection," two A. C. voltages in phase-with each other with amplitudes respectively proportional to yt+y-2yo and /3(ytyo) are obtained and applied to the two sets of plates of a-conventional cathode ray tube. A third A. C. voltage is employed to defocus the tube at one point in the A. C. cycle. The resultant indication is a straight line pointed to the indicated bearing, with the defocused spot making a tail for the arrow, thus indicating sense. By the use of suitable feedback circuits, this indicator is made stable and independent of tube characteristics. The details of the circuit arrangeinents of my invention are described more fully hereinafter. 1

The six antennas of the three-pair direction finder are arranged as in the upper-left portion of the drawing, shown at Fig. 7. Al, A2, Bl, B2,

Cl and C2 each indicate a dipole antenna with associated amplifiers as illustrated in detail in Fig. 12. In Fig. 12, elements 6 and 2 are the lower and upper halves of the dipole, 3 and d are the tubes of the untuned push-pull signal am-= plifier coupled to the signal line by transformer 5. Amplifier 3-4 is connected to the dipole antenna in push-pull so as to amplify the signal voltage set up in the antenna by. the verticallypolarized component of the signal field. Tube 5 is a horizontal pick-up amplifier connected to the mid-point of a resistor network so as to pick up the voltage between the metal sheath of the horizontal arm and the antenna. This voltage is set up by the horizontally-polarized component of the signal field. The output of 6 is coupled to the horizontal pickup line by means of transformer 1. Transformers 5 and I are designed with term-magnetic cores and close coupling to provide suiiicient signal energy transfer into their respective transmission lines over a wide range of frequencies.

The heterodyning injection voltage required in the heterodyne direction finder system here employed is supplied from the injection transmis sion' line through transformer 8 to the push-pull suppressor grids 3a and do of tubes 3 and d. This is in contrast with earlier methods in which injection voltage is induced directly into the various collector antennas from a central injection ,ontenna or from diminutive injection loops l' lac'ed near each collector antenna. The advantage of injection into the suppressor grids 3a and la" is that interference to nearby receiving installotions from this injection voltage field is elimi-' nated, Control grid bias for tubes 3, E, and 6 is suo'plie'd'by the bias lead shown connectedto an antenna, keying commutator II which blocks of! these tubes. except during those portions of the keying cycle that the particular antenna is to be active. Duringthe time this antenna amplifier is blocked oil any transfer of injection voltage to the signal line is undesirable. For this reason, neutralizing condensers 8 and [0 are provided to neutralize coupling due to the suppressor-grid to plate capacitance. i

In the upper left portion of Fig. 7 of the drawings it is seen that the signal and horizontal v pilot signal which diders in frequency by approximately 80 0. BS. from the injection voltage, excites the horizontal arms of each antenna pair longitudinally. Element EZA of'the pilotsignal commutator applies pilot signal to the horizontal. arm of the particular antenna pair active during each portion of the cycle while elements I3 and M apply pilot signal in reverse sense to the arms of the other two pairs.

In receiver i, Fig, 8, reference character i5 designates the first radio frequency amplifier tube connected to a tuned circuit coupled to the signal input line. The output of amplifier tube I5 is connected to tuned circuit iii which also receives energy from transformers i and i 8. Transformer ii is connected to the output of the main balancemontrol tubes 59 and 2E! which are excited from a tuned circuit connected to the hori zontal pickup line. This tuned circuit also ex cites tube which has a reactive plate circuit and excites the tubes of the quadrature balancecontrol amplifiers 22 and 2a in phase quadrature with it and 20. By application of controlling bias to the balance-control amplifiers, voltage from the horizontal pickup line is controlled in amplitude'and phase and mixed with the signal voltage in tuned circuit iii to balance out un wanted. horizontal pickup in t signal line.

The output of circuit i5 excites a conventional second radio frequency amplifier 2 which in turn excites a conventional first detector 25. 26 in dicates the main high frequency oscillator of the receiver and ii designates a buffer amplifier supplying oscillator voltage from 26 to 2'5. The coupling circuits shown for buffer 21 are designed for response over a wide range of frequencies to eliminate the need for tuning this stage. The intermediate frequency output of? 25 is connected to the input of the first intermediate frequency amplifier tube 28. 29 designates the second intermediate frequency amplifier tube. 38 indicates the second detector tube: 3| shows the audio frequency amplifier; and 32 designates the delayed automatic volume control amplifier.

Receiver 2 is identical with receiver i except in the arrangement of the high frequency oscillator circuits. 33 and 3c indicate the mainbalance control amplifiers; 35 the phase shifting exciter; and 36 and 3'! designate the quadrature balance-control amplifiers. 38 designates the first radio frequency amplifier; 39 shows the second radio frequency amplifier; d0 indicates the first detector: M shows the first intermediate frequency amplifier; d2 designates the second intermediate frequency amplifier; All indicates the second detector; M designates the audio frequency amplifier; and so shows the delayed autoien'c'y voltage from the oscillator in receiver 8 applied to the first detector it through the ituned buffer amplifier it and to the tuned Lifer amplifier 47. The tuned output circuit of supplies oscillator frequency voltage to the jection voltage mixer and to the pilot-signal ixer through the transmission line dd. In the injection voltage mixer, oscillator ireency voltage is applied through the untuned Lifer amplifier 49 and tuned bufier amplifier d the mixer stage 5i. To mixer stage 56 is apled also intermediate frequency voltage from c injection oscillator 52 operating near the nter of the intermediate frequency pass bands the receivers. The heterodyning injection fre .ency voltage in the output of mixer stage ii! is proximately equal to the receiver signal fre- .ency and is applied to the injection output amifier N35. The tuned output circuit of output aplifier it is coupled to the injection voltage ansmission line 53 through the injection voltage tenuator 5 3 shown in Fig. 7. The purpose of is attenuator is to adjust the strength of the m :tion voltage to approximately the same order that of the signal voltage in the outputs of the ltenna amplifiers. In the pilot-signal mixer, 55 and 5% indicate, spectively, the untuned and the tuned osciltor frequency buffer amplifiers, Elli indicates e mixer, and 58 designates the output amplifier. 1e pilot-signal mixer is identical with the in :tion voltage mixer except that the frequency the pilot-signal oscillator 58 is removed from at of the injection oscillator 52 by approximate= 80 C. P. S. The output of the pilot-signal mixer applied through the pilot-signal attenuator til own in Fig. '7 and the pilot-signal commutators A, l3 and I4, shown in Fig. 7, to the pilot-signal ductors E2 in the antenna system. If there is any coupling between the sheath curnts in the horizontal arms and the antenna sys m, the pilot signal will beat with the injection ltage to produce 80 C. P. S. components in the ceiver outputs. The vector difierence between e two 80 C. P. S. receiver outputs is indicative the components causing polarization error.

is function of the unbalance detector unit is to rive from this 80 C. P. S. unbalanced receiver tput voltage control voltages for actuating the.

lance-control tubes in the receivers to' effect reduction in this undesirable unbalanced outt. In the unbalance detector unit shown in g. 9, 6i and 62 designate amplifier tubes which ply the 80 C. P. S. difierence voltage from the o receivers to the two-stage amplifier 63-6 5. leoutput of the amplifier is applied in phase adrature to amplifiers t5 and 66 through phase litting network '61. 88 shows the main unlance detector consisting of the four diode recler elements shown, and 69 shows the quadrare unbalance detector. The two unbalance de- :tors are supplied with 80 C. P. S. reference ltages obtained by heterodyning the outputs of 2 injection voltage mixer and the pilot-signal xer in the reference voltage detector unit. vch or the unbalance .detectors G8 and iiil thus actions as a selective analyzer producing D.-C. tput depending, in magnitude and sense, on 2 80 C. P. S. component applied to its input iich is in phase with the reference voltage. [688 D.-C. automatic balance control voltages 2 applied to dill'crent llltcr condensers during a three parts of the commutating cycle by ions of control voltage commutators shown a lit) aseasas IO-and 1|. This permits the use of relatively large filter condensers resulting in stable, well filtered automatic balance control'voltages, be-

cause each condenser acquires a charge deter-- i8 is amplified by tubes 19 and 80 and applied to portional to,the component of the vector dififerthe adjustable phase delay network 8|. The output from network 8! is amplified by tube 82 and applied .to the reference voltage input terminals of the unbalance detector unit shown in Fig. 9.

The pilot-signal voltage and the injection voltage undergo different amounts of phase shift .between the outputs of their respective mixer in Fig. 8 are designed to correct this phase variation. The adjustable delay network 8| is provided to permit manual adjustment of this phase relation to supplement the action of equalizers l2 and 13 if required.

The function of the phase meter unit is to derive, from the components in the receiver outputs resulting from the beating of the signal with-the injection voltage. a. D'.-C. indicating voltage proence between these two beat notes which is in phase quadrature with their vector sum. These beat notes are applied to the phase meter ,unit

verter 9t and push-pull amplifier -96 to the other pair of input terminals of directional rec tifier 98.

The vector sum voltage and the vector difference voltage are shifted in phase ninety electrical degrees with respect to each other by the networksconsisting of resistor I38 and capacitor I31 in the sum circuit and capacitor I38 and resistor I39 in the difierence circuit.

The D.-C. indicating voltage is applied to the indicating voltage commutator 91 through the sensitivity control-98 either directly or through the neon keyer tubes 99-"!!! as determined by the selector switch I0 I. The function of the keyer tubes is effectively to disconnect the phasemeter output circuit in the absence of signal to prevent accumulated charge in the indicating voltage filter condensers 12,130 I01 from leaking off through the phasemeter circuit. I08 shows an amplifier which applies audio voltage to thaneon tubes 99-380 causing them to ignite and complete the indicating voltage circuit in the presence of si in these three tubes.

azimuth angle.

aseaese may be selected optionally by means of switch III). The purpose of this arrangement is to permit operating with a relatively large time constant when two stations are transmitting on the same channel and bearings are required on both. Selector switch IIII can then be manipulated 'manually so that each of the two sets of filter .condensers integrates the indicating voltage obtained from its respective station.

The indicator unit is shown in Fig. 10 comprls ing three A -1Q, inverter tubes III, H2 and H3 generating square wave A.-C.'voltages proportional to the three D.-C. indicating voltages, 218.311; and 21, applied to their grids. This is accomplished by the action of the keyer tube I It which periodically applies blocking bias in the cathode circuits oi the three A.-C. inverter tubes. indicates a gas triode characterized by abrupt starting of conduction. II shows another gas triode with plate connected to that of I I4 and to 'a common load resistor USA. 60 cycle per second push-pull A.-C. voltage and D.-C. bias is applied to the grids of H4 and H5 through the,

transformer shown. The function of triode I I5 is to assist in obtaining an abrupt stopping of the conduction through triode I I4. The result is that during the period when triode I I4 is non-conducting, tubes I I I, I I2 and H3 operate as conventional amplifier tubes withplates assuming potentials dependent on the D. -C. potentials Illa, ya and ye. Strict linearity in this relation is assured by the use of individual cathode degeneration resistors When triode II is conducting these three tubes are blocked off, their plate currents drop to zero, and their plate potentials quickly rise to the potential of the plate power supply (250 v.); hence an A.-C. potential of square wave form is obtained from the plate of each of these tubes which is proportional to its respective D.-C.- grid voltage. potentials are transferred to the remainder of the indicating circuits through blocking condensers IIB, II'I, III-A, H8 and H9. Resistors I20, I2I, I2I-;A, I22 and I23 are of comparative high resistance-so that the time constants of the series circuits consisting of these resistors and blocking condensers are high compared to the period of the 60 C. P. S. A.-C. wave. The result is tlTat the currents through these resistors are of saw-tooth wave form proportional in amplitude to the square'wave A.-C. voltages on the plates of the A.-C. inverter tubes. I24 and I25 indicate phase inverter tubes connected to the resistance networks shownso as to apply to the rid of amplifier I 26 a saw-tooth A.-C. voltage proportional to /3(yt-y c)" and to apply another saw-tooth A.-C. voltage to the'grid of tube I21 proportional to ya+y-2y. These voltages are those given for the denominator and'numerator, I respectively, of the equation (l/IiZ/c) which is equal to the tangent of the indicated The voltage .from tube I26 is applied through amplifier I28 to the horizontallyspaced plates of cathode ray oscilloscope I29, while the voltage from tube I21 is applied through These A.-C.

amplifier Iii! to the vertically spaced plates. In dividual cathode resistors in amplifier tubes IZIi, I25, I25, I21, I28 and I313 provide negative feedback to assure linearity in the amplifying charact-erlstics of these tubes. The indication on. the oscilloscope screen will thus be a straight line making an angle with the vertical axis equal to the azimuth angle of arrival of the received wave. Reference characters ISI and I32 indicate potentiometers used for centering the image on the oscilloscope screen for easy reading of the azimuth scale. For a sense indication, 60 cycle A.-C. voltage from the same source as that applied to lid and II 5 is applied through "phase adjusting network I 33 to the grid of gas triode I34. The condenser shown connected between the plate and ground of I35 is thus discharged abruptly once each A.-C. cycle, providing the sudden voltage surge whichis applied to one oi the grids of oscilloscope I29. This voltage surge will result in a spot on the oscilloscope image different either in focus or intensity from the remainder of the image. Phase adjusting network J33 is adjusted to make this spot appear near one end of the straight line image for the purpose of indicating sense. In the form of the indicator unit shown the output of each A.C. inverter tube .is not strictly proportional to the D.-C. voltage applied to its grid but includes a constant output which is present even when the D.-C. indicating voltage is zero. This does not cause an error in the indicated azimuth angle, however, because it merel changes the value of the arbitrary constant H in the relation 2I=H+KX+LX For the purpose of more clearly explaining the physical arrangement of the parts of the collecting system and the disposition of the cables leading to the collecting systems, I have shown in enlarged schematic plan view in Fig. 11, the layout of the cables which connect the dipole antennas with the central station; supply commutated energy to the pilot signal line; furnish injection voltage of the suppressor grids of the push pull tube in circuit with the dipoles of each of the collectors; and transmit the horizontal pick-up voltage to the central station apparatus. 1 have indicated similar reference characters on the errlarged schematic plan view of Fig. 11 to those set forth in Fig. '7. I have added symbols designating each of the parts of the pick-up system. I have shownparticularly the commutation cycle at any instance where the pilot signal commutator operates with respect to the pick-up circuit constituted by dipole antennas AI and A2 in the order I MB), I3(C), I2A(A), designating the time periods in which the attenuated pilot signal energy is supplied to the pilot signal inductors I2 through the pilot signal commutator shown at It, It and 52A in Fig. '7. The dipole antennas AI and A2 are rendered active by bias control energy supplied from the antenna keying commutator segment Ila. The order in which dipole antennas BI and B2 are supplied with inductor voltage under control of the pilot signal commutator is indicated at IMC), I3 (A) and i2A(B). The time period that dipole antennas BI and B2 are active -'is represented as controlled by bias potential sup time cycle during which the pairs of dipoles are independently effective upon the central station and the time period during which the pilot signal inductors H are effective and the relation thereof will now be fully apparent.

, While I have described my invention in one of its preferred embodiments, I realize that modifications and changes may he made and I intend no limitations upon my invention except as may be imposed by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is as follows: 1. In a direction finding system, a three pair directionally disposed antenna arrangement com-' prising radio frequency energy collecting means,

central station apparatus,- conductors leading from said central station apparatus to said radio frequency energy collecting means, amplifiers intar-posed between the ends of said conductors and the radio frequency energy collecting means con stituting each of said antenna systems, said amplifiers each including multi-grid balanced electron tube circuits, connections between certain of said grids with said radio frequency energy collecting means, a source of heterodyne injection voltage in said central station apparatus, and

means interconnecting said source of heterodyne injection voltage with others of the grids of said balanced amplifier circuits.

2. A direction finding system comprising a plurality of pairs of directionally disposed radio frequency energy collecting systems, a central station apparatus, conductors leading from said radio grids and other portions of the associated balanced electron tube amplifier circuit.

3. A direction finding system comprising a plurality of pairs of directionally disposed radio freconductor sheaths, conductors extending from the output circuit of each of said multi-grid balanced amplifier systems to said central station, a heterodyne injection voltage generator located at said central station and connected to conductors extending through said conductor sheaths withothers of the grids of said',multi-grid amplifier systems, a pilot signal conductor extending adassociated with a particular radio frequency energy collecting means is operative.

5. A direction finding system comprising a central station, pairs of conductor sheaths extendingradially from said central station, radio frequency energy collecting means located adjacent the endsof said conductor sheaths, multi-grid balanced amplifier systemshaving certain of the I grids thereof connected with the radio frequency energy collecting means adjacent the ends of said conductor sheaths, conductors extending from the output circuit of each of said multi-grid balanced amplifier systems to said central station, a hetquency energy collecting systems, a multi-grid balanced amplifier located immediately adjacent each of-said radio frequency energy collecting systems, a central station including a source of heterodyne injection voltage, conductors extending from said central station to the output circuits of each of said multi-grid balanced amplifiers, certain of the grids of said multi-grid halanced amplifiers being connected with the associated radio frequency energy collecting systems and others of said grids being connected through conductors leading to said central station with said source of heterodyne injection voltage, and means for controlling the time periods over which the balanced amplifier associated with a particular radio frequency energy collecting system is operative.

4. A direction finding systemcomprising a central station, pairs of conductor sheaths extending radially from said central station, radio frequency energy collecting means located adjacentthe ends of said conductor sheaths, multi-grid balanced amplifier systems having certain of the grids thereof connected with the radio frequency err-=- ergy collecting means adjacent the ends of said 'plifier system and with aid radio erodyne injection voltage generator located at said central station and connected to conductors extending through said conductor sheaths with others of the grids'of said multi-grid amplifier systems, a pilot signal conductor extending adjacent each ofsaid conductor sheaths, means for emitting radio frequency energy'from the ends of said pilot signal conductor in the area around said conductor sheaths, a horizontal pick-up amplifier connected with the associated balanced amplifier system, means connected with said amplifier for transmitting horizontal pick-up energy to said central station, and means for timing the excitation of said pilot signal conductor and the period over which the balanced amplifier system associated with a particular radio frequency energy col lecting means is operative.

6. A direction finding system. comprising a central station, pairs of conductor sheaths extending radially from said central station, radio frequency energy collecting means located adjacent the ends of said conductor sheaths, multi-grid balanced amplifier systems having certain of the gridsvthereof connected with the radio frequency onergy collecting means adjacent the ends of said conductorv sheaths, conductors extending from the output circuit of each of said multi-grid balanced amplifier systems to said central station, a heterodyne injection voltage generator located at said central station and connected to conductors extending through said conductor sheaths with others of the grids of said multi-grid amplifier systems, a pilot signal conductor extending ad- .iacent each of said conductor sheaths'meahs for emitting radio frequency energy from the ends of said pilot' signal conductor in the area around said conductor sheaths, a horizontal pick-up amplifier having its input circuit connected with the input circuit of the associated balanced amfrequency en= ergy collecting means and having its output circuit connected through 'a horizontal pick-up line with said central station, and means for timing the excitation of said pilot signal conductor and the potential applied to both said horizontal pickup amplifier and the associated balanced amplifier system.

tralstation. pairs of conductor sheaths extending radially from said central station. radio frequency energy collecting means located adjacent the ends 01 said conductor sheaths, multi-grid balanced amplifier systems having certain of the grids thereof with the radio frequency energy means adjacent the ends of said conductor sheaths, conductors extending from the output circuit of each or said multi-grid balanced amplifier systems to said central station, a het-v transmitting signal energy to said central station,

a horizontal pick-up amplifier associated with each balanced amplifier system, a horizontal pickup line extending through said conductor sheath from the output of said horizontal pick-up amplifier to said central station, a bias potential controlconductor extending through said conductor sheaths from said central station apparatus to both said horizontal pick-up amplifier and said balanced amplifier system, and means for timing the period over which the balanced amplifier system associated with a particular radio frequency energy collecting means is operative and timing the application of bias potential toboth said horizontal pick-up amplifier and, said, bal anced amplifier system.

-8. In a direction finding system, a three pair antenna directionally disposed arrangement comprising radio frequency energy collecting means, a two channel signal receiving system, conductors subject to unwanted horizontal pick-up energy establishing connections between said radio frequency energy collecting means and the input circuits of said two channel signal receiving system, a visual indicator operated by the conjoint output of said signal receiving channels, and means controlled by the energy received from said radio frequency energy collecting means for balancing out imwanted horizontal pick-up energy in the conductors which interconnect said radio frequency energy collecting means with said signal receiving channels.

9. In a direction finding system, a three pair directionally disposed antenna arrangement comprising radio frequency energy collecting means, a pair of signal receiving channels, conductors exposed to imwanted horizontal pick-up energy ductor sheath and establishing connection be tween said radio frequency energy collecting means and said signal receiving channels, visual indicator means for integrating the output energy from both of said signal receiving channels, unbalance detector means connected with the output of said signal receiving channels, and commutating means for successively controlling the effect of said radio frequency energy collecting means upon said signal receiving channels, and means controlled by said unbalance detector means for balancing out horizontal pick-up energy incident upon said conductor sheaths and said conductors.

11. In a direction finding system, a three pair directionally disposed antenna arrangement comprising radio frequency energy collecting means,

a twochannel signal receiving circuit, a visual indicating'system, and circuits for rendering effective on said visual indicating system the illtegrated eflects of energy incident upon said three pair antenna arrangement through said two channel signal receiving circuit.

12. In a direction finding system, a three pair directionally disposed antenna arrangement, a two channel signal receiving circuit connected with said three pair antenna arrangement, a visuai indicator including a cathode ray tube hav ing cathode ray indicating means therein, and three control circuits for operating said cathode ray tube from the output of said two channel signal receiving circuit in accordance with energy collected by said three pair antenna arrangement,

interconnecting said radio frequency energy 001- lecting means and said pair of signal receiving channels, a visual indicator operated by the integrated output oi. said si nal receiving channels, commutating means for rendering said radio frequency energy collecting means selectively effective with respect to said signal receiving channels, and means controlled by the energy received by said radio frequency energy collecting Bit means for balancing out unwanted horizontal pick-I11 energy in the said conductors.

10. In a direction finding system, a three pair directionally disposed antenna arrangement comprising radio frequency energy collecting means, a pair of signal receiving channels, conductor sheaths ,between said radio frequency energy collecting means and said signal receiving receiving circuit in accordance with energy col lected by said three pair antenna arrangement, and commutating means for successively render" ing individual pairs of said three pair antenna arrangement eilective upon two channel sig nal receiving circuit.

l-l In a direction finding system, a three pair directionally disposed antenna arrangement com" prising radio frequency energy collecting means, a multi-grid balanced electron tube amplifier having the input circuits thereof connected with said radio frequency energy collecting means, conductors extending from the output circuits of said balanced electron tube amplifiers, a pair of signal receiving channels having their input cir cults connected with said conductors, a condoctor sheath enclosing said conductors, a visual indicator operative by the combined effects of output currents delivered by said pair of signal receiving channels, means for introducing a h'eterodyne injection voltage into the grids of each of said multi-grid balanced amplifiers, means for introducing a pilot signal voltage to the said sheaths for transmission to said pair of signal receiving channels, and means for timing the periods of application of said pilot signal voltage.

15. In a direction finding system employing a three pair directionally disposed antenna ar rangement, each coupled through balanced amplifier systems with a two channel signalling system having the output thereof connected in comchannels, conductors extending through said can mon to a visual indicator circuit, means for inte= rating the effects of signalling energy received by the three pair antenna arrangement, means for transmitting to each of the balanced amplifler systems commutcted. bias energy for successively rendering the pairs of unteunocrrangementc reflective upon the signal receiving channels three sets of integrating condensers, and" means for connecting the three sets of inmgrating condensers to the visual indicctor circuit in synchronism with. the commutation of sold bias energy.

mm c. direction finding system employing a three pair directionally disposed M rongement, ecchcoupled through balanced cm- I plifier systems and sheathed conductors with a two channel signalling system having the output thereof connected in common to c visual in- 1 dicator circuit, moons foriutegrslting the effects of signalling energy received by three pair antenna arrangement, means for trerisriiitting to each of the balanced umpliflerisystems commutated bias energy for successively rendering the pairs of antenna, orrangements eiiective upon the signal receiving channels, three sets of inte rating condensers, a generator of alternating cur-. rent,- means for transmitting the alternating current to the balanced amplifier systems, means for transmitting alternating current of o Irequency different from the frequency of the afore-.

mentioned alternating current for establishing a pilot signal field about the sheaths enclosing the conductors that extend, between the balanced amplifier systems and the signal receiving channels, and mecca for connecting said three sets of integrating condensers to cold visual indicator circuit in syrichronism with the commutation of said bios energy 17,111 2} direction finding system which includes a three pair directionally disposed antenna-1 arrangement constituted by radio frequency energy collecting" means connected through balanced cmpliflers and sheathed conductors with a two chennelsignal receiving system having the outputs of the two channel signal receiving system integrally connected with a visual indicator.-

the method of balancing out unwanted horizontal pick-up energy which comprises electronically rendering the antenna arrangements successively eflective to impress signalling energy upon said signalrecelving channels, differentially controlling the impression of a pilot signal alternating current field around the sheathed conductors and controlling the relative time periods of effective impression of the pilot signal voltage with respectto the periods of energizetion of the pairs of antenna arrangements.

18. In a direction finding system, a. three pair directionally disposed collector arrangement, each collector arrangement being connected through.

a balanced amplifier system and sheathed condoctor with a receiving system, a pilot signal sys-,

tem including means for impressing a pilot signal upon said sheathed conductor, separate amplifier means connected with said collector orrangements and connected with said receiving system for impressing signal energy; and incidental horizontal pick-up energy induced in saidsheathed conductor, upon said receiving system, an unbalance detector in said receiving system for detecting the presence ofpilot signal energy,

and means controlled thereby for controlling the t application of horlzontaLpick-up energy to the input circuits of the signal receiving system for balancing out unwanted currents while effectively receiving the signal energy.-

WALTER H. WIRKLER. 

